Due to their long lifetime, small size, high resistance to shock and vibration, low heat generation, and low power consumption, light emitting diodes (LEDs) have been widely used in displays or as light sources in various consumer electronic devices, electrical appliances, and apparatuses. Recently, multi-color and high brightness LEDs have also been used in, for example, outdoor, large-screen displays and traffic lights. In the future, LEDs may act as the major illumination light source, with the advantages of energy savings and as environmental protection.
One advantage of using LEDs as a lighting source is that it can be designed to generate white light with variable correlated color temperature (or simply color temperature, “CCT”). One conventional color-temperature-tunable device is a white LED comprising a blue LED chip and a yellow LED chip, emitting blue light and yellow light, respectively, when driven by driving currents. To change the color temperature of such a device, one can change the ratio of the driving currents for the blue and yellow LED chips to change the ratio of the intensities of the blue and yellow lights. However, since the luminous efficiency of blue LED chips and yellow LED chips is different, changing driving currents to change the ratio of blue and yellow lights may result in wasted power. The brightness of the device also can change dramatically. Moreover, in such a device, lights having different wavelengths (i.e., lights having different colors, such as blue light and yellow light) are emitted from two LED chips spatially arranged apart from each other, and the resultant white light may not be spatially uniform, especially when viewed from a position close to the white LED. Therefore, additional components may be needed to minimize such spatial non-uniformity, so as to realize more uniformly white light.
A widely accepted approach to realize white LEDs is to use the combination of a blue LED chip and a yellow phosphor. The yellow phosphor will absorb a portion of the blue light emitted by the blue LED chip, and in turn emit yellow light. Unabsorbed blue light mixes with the yellow light to generate a white light. One approach to realize color temperature tuning in such a white LED is to add a red or yellow LED chip. By changing the ratio of the driving currents for the blue LED chip and the red or yellow LED chip, the color temperature of the resultant white light can be changed. However, the resultant white light may also not be spatially uniform since the blue LED chip and the yellow LED chip are spatially arranged apart from each other, and thus additional components may be needed to correct this non-uniformity.
Therefore, there is a need for a color-temperature-tunable device, which emits a more uniform white light having different color temperatures and operable in a more power-efficient way.